Research On Main Circuit Parameters Design And Circulating Current Control Of Modular Multilevel Converter

With the deterioration of the environmental pollution and energy consumption issues, renewable energy power generation and related power transmission technology has been rapid development. High voltage direct current transmission provides a good solution to adjust the unbalance distribution of resources and clean energy power transmission. Modular multilevel converter has been widely used in high voltage DC transmission and other fields because of its DC bus, low output harmonic content, lower switching stress. In this paper, the work mechanism, modulation strategy, system implementation and related control strategies of the modular multilevel converter are studied.Firstly, the basic working principle and research hotspots of MMC is presented. The equivalent mathematical model of MMC is established. The realization principle, comparisons and feasibility used to MMC of several kinds modulation strategies is studied.Based on the mathematical model established, the paper proposed the design method of the arm inductor from four aspects which are avoiding system resonance, meet the operating condition of power, circulation suppress and restrain the short-circuit current of DC bus. The selection criteria of submodules capacitor is proposed based on the relationship between voltage fluctuation and capacitor size. the sub module capacitor of a three-phase MMC simulation model is redesigned based on the criteria, the simulation results verify the correctness of the theory. In order to do the comparison of submodules capacitor between different MMC projects, the the principle of Unit capacity discharge time constant is introduced, the reference range of the submodules capacitor of the MMC engineering is given.Finally, the suppression methods of circulating current harmonics in MMC are studied. The causes and main components of the harmonics are analyzed by building the accurate mathematical model of circulating current. On the basis of analysis, the circulating current harmonic suppression methods based on resonant control and repetitive control are proposed, then two controllers are designed correspondingly. The circulating current control algorithm are run on the simulation model and the prototype, the simulation and experimental results verify the effectiveness of the proposed control methods.